There are a number of patents which disclose specific compositions which reduce or eliminate this foam. For example, U.S. Pat. No. 5,192,336 assigned to Nalco, discloses that bis-strearamides act as defoamers by remaining dispersed within the fuel. This is in contrast to silicone polyether defoamers, as claimed in U.S. Pat. No. 4,690,668 assigned to Dow Corning and in U.S. Pat. No. 3,233,986 assigned to Union Carbide, which because of their higher density relative to hydrocarbon fuel settle out of the fuel over time, potentially rendering them less effective without periodic agitation to re-disperse them. Silicone polyethers also can suffer from the fact that they tend to be more soluble or dispersible in water, a constant component of hydrocarbon fuels. In storage tanks, water tends to coalesce, forming a layer at the bottom of the tank. As the silicone polyether settles and its insolubility in the hydrocarbon, eventual contact with the water layer can result in its being absorbed into that phase, thus irreversibly removing it from the fuel entirely.
Current middle distillate fuels exhibit foaming during transfer operations, such as filling a vehicle's fuel tank at a filling station. In the processing, transportation and storage of hydrocarbon liquids, it is frequently observed that foaming occurs as the liquid is passed from one vessel to another. For example, as liquid hydrocarbon fuel is passed quickly into a storage tank a foam may develop at the surface of the fuel and, in many cases, the extent of foaming is sufficiently significant and persistent to require a reduction in the rate of passage of the liquid fuel into the vessel. It is highly desirable to provide means for controlling foaming so as to permit high rates of passage.
Various proposals have been made for controlling foaming of various grades of hydrocarbon liquids by use of additives for example silicone products. However, the reduction of foaming of diesel fuels by use, in extremely small quantities, of technically acceptable silicone additives has not been satisfactorily solved prior to this invention.
U.S. Pat. No. 3,233,986 is concerned with siloxane polyoxyalkylene block copolymers as antifoam agents and discloses the use of a wide variety of such copolymers to reduce the tendency of organic liquids to foam. Organic liquids mentioned in said specification include inter alia various hydrocarbon liquids including liquid hydrocarbon fuels e.g. kerosene, gasoline and diesel fuel. Among the many copolymers advocated in said specification are those comprising groups represented by the formula: ##STR1## wherein G.sup.3 is a member selected from the group consisting of the hydrogen atom and the monovalent hydrocarbon groups, G" is an alkylene radical containing at least two carbon atoms, G' is a divalent hydrocarbon radical, G is a monovalent hydrocarbon radical, n has a value of at least two and c has a value from 0 to 2 inclusive.
U.S. Pat. No. 3,233,986 teaches that at least 60% by weight of the groups OG" must be oxyethylene or oxypropylene groups and that other oxyalkylene groups may be present in the groups OG". It is said that each oxyalkylene block preferably contains from four to thirty groups OG". The specification teaches that the number of oxyalkylene groups (OG") and that part of the average molecular weight of the copolymer that is attributable to the oxyalkylene blocks is not critical, and that useful copolymers can contain siloxane blocks and oxyalkylene blocks in any relative amount.
U.S. Pat. No. 3,233,986 states that the amount of the copolymers used with a liquid hydrocarbon is not critical and can range from 5 to 2000 parts by weight of the copolymer per million parts of the liquid.
Some copolymers of the type disclosed in U.S. Pat. No. 3,233,986 when employed in certain hydrocarbon fuels, particularly when used in amounts of less than 100 parts copolymer per million parts hydrocarbon, do not act to reduce the tendency of the hydrocarbon to foam but rather to increase the tendency of the hydrocarbon to foam.